1. Field of the Invention
This invention relates to a fluid flow meter and system for using same, and specifically to a flow meter formed by micromachining and having a channel with portions of different cross-sectional area, and including a pulsed heating element and two spaced-apart downstream heat sensors for precisely measuring fluid velocity.
2. Description of the Prior Art
It is known to introduce heat into a fluid stream to measure fluid flow. One such device includes two heating elements placed in the fluid stream; both elements are electrically heated and cooled by a stream of fluid. The upstream element is cooled by the fluid stream more than the downstream element, and the measured temperature difference between the two elements indicates flow.
Another method employs a heating element, a temperature sensor upstream from the heating element, and a temperature sensor downstream from the heating element. Fluid passes by the upstream sensor and is then heated by the heating element, while the heated fluid continues on to the downstream sensor. The measured temperature difference between the upstream and downstream sensors determines flow rates. Both the two and three element configurations described above have been formed using semiconductor micromachining technology.
To accurately measure flow using these techniques without calibration, the specific heat and thermal conductivity of the fluid and the various components of the flow meter must be known precisely. Moreover, the effects of thermal conductivity vary with changes in flow rate, ambient temperature, fluid temperature, fluid type, and fluid concentration. One method of compensating for thermal conduction losses includes adding another sensing element in a closed off channel to compensate the device for thermal conduction losses. In a gas application, this is relatively easy; however, in a liquid application, it is difficult to ensure this dead-end channel becomes primed.
Other thermal flow meters overcome some of the above disadvantages. One such approach (Miller, Jr. et al., U.S. Pat. No. 4,532,811) applies a thermal pulse to a stream of fluid and has a single downstream heat sensor to sense the thermal pulse. The transit time between the heating element and the heat sensor determines flow velocity. The Miller thermal pulse technique is effective over a wide range of fluid temperatures, because the unheated fluid is used as a reference: the downstream sensor detects thermal pulses, i.e. envelopes of fluid traveling through the flow channel that are warmer than the unheated fluid. Therefore, the thermal pulse technique is advantageously insensitive to changes in ambient temperature.
A major disadvantage of Miller's approach is that there is a delay associated with the transfer of heat to and from the fluid. This delay is associated with the thermal masses, thermal conductivities, and heat-transfer coefficients of the heating element, sensor, and fluid, and must be accounted for when calculating flow rates. Since the delay is related to the properties of the fluid, the flow meter inconveniently must be recalibrated for different types and concentrations of fluids.